This type of liquid level sensor having a plunger core is already known in which the graduated flexible metal tape that is windable into a sealed storage housing outside the tank penetrates into the tank through a passage provided with means for sealing relative to the outside environment. The free end of the tape is fixed to a plunger core which is provided with a detection feeler for detecting the presence of liquid and with means for sending an information signal to a receiver situated outside the tank and provided, for example, with an indicator light that lights up when the detection feeler comes into contact with a liquid. The operator thus causes the graduated tape to be paid out until observing that the indicator lamp is in a boundary region between switching on and switching off, which means that the detection feeler of the plunger core is situated at the interface between a gaseous atmosphere and the liquid. When the indicator light indicates that the detection feeler has reached the interface which it is to find, then the operator observes the graduation on the tape as situated adjacent a reference mark formed in an observation window, which graduation provides an indication of the position of the interface that is to be checked, i.e. an indication of the level of the liquid in the tank.
In practice, in industry, liquid level sensors using plunger cores are used for detecting liquids of a wide variety of types, some of which can be highly aggressive, chemically. At present, each sensor is adapted to a liquid of a particular kind and is therefore not universal. Thus, for certain types of liquid, a detection feeler is used that responds to variations in conductivity, whereas for other types of liquid a detection feeler is used that is of the capacitive type, and for yet other types of liquid, a detection feeler is used that is of an optical type. In addition, optical or capacitive type detectors are somewhat fragile and therefore difficult to use.
Also known are liquid presence detectors of the type based on waveguides for elastic waves (or "sound"). Nevertheless, such detectors are generally rather bulky, of limited sensitivity, and mounted at a fixed station on the wall of the tank containing the liquid.